Rapid self-heating synthesis of Fe-based nanomaterial catalyst for advanced oxidation

Iron-based catalysts are promising candidates for advanced oxidation process-based wastewater remediation. However, the preparation of these materials often involves complex and energy intensive syntheses. Further, due to the inherent limitations of the preparation conditions, it is challenging to realise the full potential of the catalyst. Herein, we develop an iron-based nanomaterial catalyst via soft carbon assisted flash joule heating (FJH). FJH involves rapid temperature increase, electric shock, and cooling, the process simultaneously transforms a low-grade iron mineral (FeS) and soft carbon into an electron rich nano Fe0/FeS heterostructure embedded in thin-bedded graphene. The process is energy efficient and consumes 34 times less energy than conventional pyrolysis. Density functional theory calculations indicate that the electron delocalization of the FJH-derived heterostructure improves its binding ability with peroxydisulfate via bidentate binuclear model, thereby enhancing ·OH yield for organics mineralization. The Fe-based nanomaterial catalyst exhibits strong catalytic performance over a wide pH range. Similar catalysts can be prepared using other commonly available iron precursors. Finally, we also present a strategy for continuous and automated production of the iron-based nanomaterial catalysts.

S4 the standard procedures using the ATHENA module implemented in the FEFIT software packages 2, 3 . The k 3 -weighted Fourier transform (FT) of x (k) in R space was obtained over a range of 0-14.0 Å by applying a base window. Raman spectroscopy (LabRam HR Evolution) was performed to analyze the degree of graphitization. Nitrogen adsorptiondesorption isotherms and surface areas were acquired by a BET analyzer (Quadrasorb SI, America). The elemental (C, H, N, S) compositions were measured by an elemental analyzer (Vario ELIII, Germany Elemental Instrument Co., Ltd). O element composition was acquired by the difference. The measurement of ash content was referred to literature 4 .

Degradation of chloramphenicol
CAP degradation in aqueous solutions: Degradation of chloramphenicol was performed in a batch reactor. A Fe-based material (25 mg) and sodium peroxydisulfate (41.7 mg) were added to a centrifuge tube with 25 mL chloramphenicol solutions (60 ppm), and the initial pH is about 3. The Fe-based material or sodium peroxydisulfate was added independently into a centrifuge tube with 25 mL chloramphenicol solutions (60 ppm) as control. After that, the centrifuge tube was put in an oscillation box with 150 rpm at 28 o C.
The solutions were taken at a specific time, filtered through a 0.22 μm filter, and added immediately with an equal volume of methanol to prevent a reaction before measurements. The concentrations of chloramphenicol were detected rapidly by Highperformance liquid chromatography (HPLC) at a wavelength of 278 nm, 1 mL min -1 mobile S5 phase (the volume ratio of methanol to ultrapure water is 4:6), and the column temperature is 25 o C.
CAP degradation in the soil slurry: Degradation of chloramphenicol was performed in a batch reactor. A Fe-based material (25 mg), sodium peroxydisulfate (41.7 mg), and soil (2500 mg) were added to a centrifuge tube with 25 mL chloramphenicol solutions (60 ppm) without regulating pH. The soil and sodium peroxydisulfate were added into a centrifuge tube with 25 mL chloramphenicol solutions (60 ppm) without Fe-based material as control.
After that, the degradation process is the same as CAP degradation in aqueous solutions.
The degradation process of chloramphenicol in three types of soil (red soil, yellow soil, and black soil) were conducted.

Cyclic voltammetry and electrochemical impedance spectroscopy measurements
A Fe-based material coated glassy carbon electrode (using Nafion perfluorinated resin solution as binder) was selected as the working electrode. Ag/AgCl electrode and Pt electrode were used as reference and counter electrodes, respectively. A mixed solution of sodium peroxydisulfate (7 mM) and sodium sulfate (0.2 M) was used as an electrolyte.
The cyclic voltammetry curve was recorded at the potential range from -1.1 to 0.2 V. The scan rate was 10 mV s -1 . Electrochemical impedance spectroscopy was also measured.
Low frequency and high frequency were 0.01 and 100000, respectively.

S6
Detection of iron species: Total dissolved iron and Fe (II) concentrations in the degradation process were detected using the standard method (HJ/T 345-2007).
Generally, filtered samples (1 mL), 1,10-phenanthroline (2 mL, 0.5 wt%), and a buffer solution of acetic acid and ammonium acetate (5 mL) were mixed. After that, the mixed solution was diluted to 50 mL with deionized water. Finally, Fe (II) concentrations were detected by a UV-vis spectrometer at 510 nm. For the total iron concentrations, a similar technique as above was employed except that hydroxylamine hydrochloride (1 mL, 0.5 g mL -1 ) was also added to the mixed solution.
Radical quenching tests: Radical quenching experiments were also tested. Tert-butanol (TBA) was a strong quencher of ·OH, methanol was a strong quencher of ·OH and SO4 ·-, and potassium iodide was a quencher of surface radical of Fe-based material.
Quantitative analysis of hydroxyl radical: the measurement was conducted using benzoic acid as a probe. After the reaction of ·OH with benzoic acid, p-hydroxybenzoic acid (p-S7 HBA) was produced and its concentration was measured by HPLC. The method and calculation of cumulative ·OH concentration were referred to literature 6 as equation (1).

Intermediate detection by GC-MS and HPLC-QTOF-MS
After the CAP degradation process of 15 min, the reacted solution (50 mL) was extracted with dichloromethane (5 mL). The intermediate products were detected by GC-MS. The detailed information can be referred to this literature 7 . HPLC-QTOF-MS with an ESI source in the negative ionization was used for analysis. An Agilent Eclipse C18 column (3.0 mm x 150 mm, 1.8 μm) was used for separation. An injection volume was 3.0 μL and acetonitrile with 0.1% formic acid was selected as the mobile phase at a flow rate of 3 mL min -1 . The m/z range was 50~1500. The capillary voltage of the ESI was set at 3500 V, the gas temperature was set to 325 o C, the drying gas flow was 12 L min -1 , and the nebulizer was 60 psi. The inorganic substances of chloride and nitrate were measured by ion chromatography (ICS-5000).

Gas production detection from the FJH process by GC-MS and HPLC
The CS2 and S2 gas from FJH processes were collected by ethanol and CS2 solvent condensation according to the literature 8  In addition, the PDS activation mechanism of Fe-based materials generally involved PDS adsorption on metal sites and the subsequent reduction of PDS to produce reactive free radicals for CAP degradation. The Fe 0 /FeS heterostructure embedded in thin-bedded graphene was the main contributor to CAP degradation. Therefore, the system of PDS adsorption on Fe 0 /FeS heterostructure embedded in thin-bedded graphene (Fe 0 /FeS/C or FeS/Fe 0 /C) was structured for DFT analysis. To explain the superior activation performance of Fe 0 /FeS heterostructure on PDS, we compared FeS embedded in thinbedded graphene (FeS/C) and Fe 0 embedded in thin-bedded graphene (Fe 0 /C). The role S9 of thin-bedded graphene was further explained by structuring the FeS (FeS without graphene) and FeS embedded in thin-bedded graphene (FeS/C) respectively.
Meanwhile, the reduction of PDS on the material to produce reactive free radicals was testified, which produced the Na2SO4 and ·OH due to the O-O bond breakage of Na2S2O8. Therefore, the PDS activation reactions on Fe 0 /C and Fe 0 /FeS/C from the beginning to the end were constructed to evaluate the energy barrier for the O-O bond breakage and Gibbs free energy for the whole reaction process. super cell of Fe and FeS was structured for DFT calculation. To eliminate the interactivity of the adjoining slab model, the vacuum layer was set to 15 Å. The cutoff energy for the plane-wave-basis was set to be 500 eV. The k-point sampling grid was set to 3×3×1. The convergence tolerances of energy and force were set to 1.0 x 10 -4 eV atom -1 and 1.0 x 10 -2 eV Å -1 , respectively. The DFT-D3 method was used to describe the van der Waals S10 (vdW) interactions between substrates and adsorbate 13 . The adsorption energy (Ead) of Na2S2O8 on heterostructure can be calculated by equation (2): Where, E (sub+Na2S2O8) and E (sub) are the total energy of the FeS (110)/C or Fe (011)/C heterostructure with and without Na2S2O8, respectively, and E (Na2S2O8) is the energy of Na2S2O8.
The transition state of O-O bond breakage in the adsorption configuration of Fe 0 /C and Fe 0 /FeS/C with PDS was calculated according to the reacted equation as equation (3).
The transition state method was CINEB.

G = Etot + Ezpe +T ΔS (5)
Etot represents the total energy of the optimized structure. T was 298.15 K, Ezpe represents the zero point vibration energy, and ΔS represents enthalpy change.

Automation equipment
For large-scale production of superior Fe-based material, the automation equipment was designed. The automation equipment includes a loading and collecting area, robot arm, and reacted technics area. After loading, the raw material was transferred into the reacted S11 area by the robot arm. Subsequently, a current was driven in the material by the triggered voltage, and a reaction was achieved to improve the material structure as a superior Febased material. Finally, the robot arm further transfers Fe-based material from the reacted technics area to the collecting area. After completion, the next steps are circulatory to continuously produce Fe-based material.